I might have no idea what I'm saying but here goes. n = 5. Since the 3 X 3 grid is chosen from the 5 x 5 grid or 3 x 3 grid is chosen from the n x n grid. If g = 11, and x = 1, then x+2n or 1+2(5) = 11. And if i = 13. Then x+2+2n or 1+2+2(5) = 13. Since cg-xi = 20, and n = 5, then 4n or 4(5) equals 20.

start from the point "x" in an nxn grid.
going "down one" will increase the number by n
going "right one" will increase the number by 1

I have a small doubt....
so now this formula (x+2n) can give me g in any grid, lets substitute this formula for a 3x3 grid. where n=3
1+3(3)=7, but it should be 11, why doesn't this formula work this way, it only works for the 5x5.